ESR Identification of Slater Configurations and of Exchange of the (AsO4)4− Radical in Irradiated Ferroelectric KH2AsO4 and KD2AsO4 and Antiferroelectric NH4H2AsO4 and ND4D2AsO4

The ESR study of the g̃ spectroscopic tensor and the As hyperfine tensor T̃_As of the (AsO₄)⁴⁻ radical in crystals of ferroelectric KDA and DKDA and antiferroelectric ADA and DADA shows that it is possible to identify selectively the Slater configurations of the radical in these crystals. In the paraelectric phase of these crystals we observe dynamic low frequency fluctuations of g̃ and T̃_As which correspond to an exchange between the ``upper'' and ``lower'' Slater configurations of the radical. The temperature dependence of the correlation time of this process is close to Arrhenius laws: ${\tau }_c{\mathrm{=\tau }}_{\text{c0}}\hspace{0.17em}\exp {\mathrm{(E}}_a\mathrm{/kT)}$ with E_a=0.21 eV for KDA, 0.27 eV for DKDA, 0.33 eV for ADA, and 0.43 eV for DADA. For the ammonium ADA and DADA salts, the passage through the Curie point shows drastic changes of the spectra of this radical. The analysis of the lines shows that they correspond to several distinct new Slater configurations of the radical. Our general conclusion is that there exists a perturbed zone around the (AsO₄)⁴⁻ radical which exhibits several different features characteristic of local symmetry, local order, and, with regard to the paraelectric phase, local dynamics. The question of antiferroelectric order in ammonium salts is also discussed with reference to the Nagamiya model.